Screen for fiberizing bushings

ABSTRACT

A bushing for fiberizing molten material, such as molten glass, having a screen mounted in the bushing spaced above the tip or orifice plate with a central portion of the screen having a significantly lower percent of hole area than the percent of hole area in end portions of the screen. This bushing improves fiberizing efficiency in channel positions of a fiberizing operation. Also, such a screen can be laid on top of a conventional screen to convert a normal bushing to a channel position bushing. Methods of using these types of bushings to improve fiberization in the channel positions and for modifying conventional bushings for other uses are also disclosed.

BACKGROUND

[0001] The present invention involves an improved bushing apparatus formaking glass fibers and an improved method of making and using glassfiberizing bushings.

[0002] In the manufacture of continuous fibers from a molten materiallike molten glass, the molten material is often generated by a tankfurnace and distributed to a plurality of fiberizing bushings via one ormore channels and one or more bushing legs connected to the channel(s).Each bushing leg comes off the channel at about 90 degrees and containsa plurality of bushings that are spaced apart. The molten materialexiting the tank furnace into the channel(s) is much hotter than desiredfor fiberizing and the molten material entering the bushing legs istypically hotter than desired for fiberizing, particularly when thefurnace is being run close to designed capacity.

[0003] Precious metal bushings for making continuous glass fibers arewell known, having been in use for more than 50 years. Many types ofbushings exist for converting molten glass into continuous glass fiberand products. Typical types of bushings are shown in U.S. Pat. Nos.3,512,948, 4,155,732, 4,272,271 and 4,285,712, the disclosures of whichare hereby incorporated by reference. All the bushings shown in thesepatents teach the use of a perforated plate or screen, welded to the endwalls and side walls at some distance above a tip plate containinghundreds of nozzles or tips where molten glass, as it emerges from theorifice of each tip, is converted to a continuous glass fiber in a knownmanner. These patents teach that the purpose of the screen is tocondition the glass, homogenizing the chemistry and temperature, and toprevent pieces of refractory or unmelted glass from reaching the tipplate. Most, if not all, of these references teach using a uniform holepattern with uniform hole size over the entire surface of the tip plate.Normally, the screens taught by these references improve the temperatureprofile of the tip plate, i.e. produce a more uniform temperature in themolten glass just above the tip plate in all directions than if thescreen were not present.

[0004] These bushings work well as long as the molten material enteringthe bushings is fairly uniform in temperature, but often there is atleast a streak of molten material in the flow that is significantlyhotter than the molten material next to the walls of the channel. Thishotter material has a lower viscosity than the cooler material next tothe walls. When it enters the bushing, always in the first position nextto the channel and sometimes in the second position of a bushing leg, itflows through holes in a conventional screen in the bushing faster thanthe cooler material. This causes the temperature profile of the tipplate spaced below the screen to be non-uniform. When this happens, agenerally central portion of the length of the bushing tip plate runsconsiderably hotter than the ends. This hotter central portion can beoffset to the down stream end due-to the velocity vector of the hotterstream of glass. The hot glass has a higher velocity down center of thebushing leg and down the orifices to the bushings than the colder glassnext to the walls and bottom.

[0005] The first position in each of the legs, the positions next to thechannel, are called channel positions. The channel position in each leghas the most glass passing over it than any of the remaining bushings inthe leg, and the velocity of the molten glass passing over the channelpositions can be significantly higher than it is further down the leg.When hot glass dives into the orifices feeding the channel positions, itsubstantially increases the break rate of the bushing and also increasesthe variation of the fiber diameters of the fiber coming from thebushing due to the higher temperature gradient this condition causesacross the tip plate.

[0006] The use of a screen having a non-uniform hole size and/or holedensity is taught by U.S. Pat. No. 4,612,027, but this reference doesnot suggest using that screen for addressing the above describedproblem. This patent teaches making a bushing having a dripless tip areaand a dripping tip area in the tip plate of the bushing. The bushingtaught by this patent has a screen that has much less resistance to flowin the center portion of the screen than the portion or area adjacenteach end of the screen, i.e. the center portion of the screen has muchlarger holes and/or a higher hole density than the areas of the screenadjacent each end of the bushing. Also, the bushings taught by thispatent must have vertical walls extending from the top of the tip plateto the bottom of the screen to separate the areas of different rates ofmolten glass flow to function as taught. Nothing in this patentdiscloses or suggests a solution to the problem of bad tip temperatureprofile in channel positions and the negative results this conditioncauses. Instead, this patent accepts that the tips on the outerperiphery break out more frequently and teaches a bushing that toleratessuch frequent fiber breaks and lower fiberizing efficiency.

SUMMARY OF THE INVENTION

[0007] It has been discovered that if the hole density, number of holesper unit area, and/or the hole diameter, is decreased in the screen inthe area above where the hotter glass flow negatively affects the tipplate temperature profile, the temperature profile of the tip plate isgreatly improved. This discovery has led to bushings for the channelpositions that allow fiberization in the channel positions with muchimproved efficiency, compared to using conventional bushings in thesepositions, and similar efficiency to other positions. This inventionalso reduces the fiber diameter variation of the fibers produced fromthe channel positions. However, these bushings, so designed and builtfor the channel positions, are often not acceptable for non-channelpositions because the tip plate of this bushing has a bad temperatureprofile in all positions except the channel positions and sometimes inthe second position from the channel. When the reduced hole area portionof the screen is placed in the center lengthwise and is not offsetsignificantly, the bushings will also perform satisfactorily innon-channel positions, but this type of bushing will sometimes notoptimize channel position performance.

[0008] The present invention includes a bushing for making fibers from amolten material comprising at least one sidewall and a tip plate ororifice plate through which molten glass flows to form the fibers, and ascreen having a plurality of holes (orifices) therethrough and mountedon the interior of the bushing and spaced above the top of the tip plateor orifice plate, said screen having holes therein and being attached tosaid sidewall, the improvement comprising a generally mid or centralportion of the screen having a hole area per unit area of screen that issmaller than the hole area per unit area of screen of end portions oneither side of the mid or central portion. The present invention alsoincludes the method of making this inventive bushing and of using thistype of bushing to make fibers including using this bushing in thechannel positions.

[0009] While this solution solves the channel position tip platetemperature gradient problem, using a different bushing design in thechannel positions than is used in the non-channel positions requiresthat many more bushings be inventoried. This is aggravated by needingtwo types of the bushing made according to the present invention inaddition to the conventionally designed bushing. Two types of bushingsare needed to maintain the same front to back orientation of thebushings on both sides of the channel. The same orientation is importantto fit auxiliary equipment for the bushing like water cooling lines andthermocouple connections. The area of the screen where smaller and/orfewer holes/unit area are placed for decreased flow on channel positionsis frequently not in the center of the bushing, lengthwise, so bushingsdesigned for the channel position on one side of the channel will notwork for the channel positions on the opposite side of the channel.These bushings are made from precious metal alloys, such as 78 percentplatinum and 28 percent rhodium, which are more expensive than gold.This increase in bushing inventory and precious metal inventoryrequired, compared to a situation where there is only one type ofbushing, ties up substantial additional valuable capital assets from useelsewhere in the operation.

[0010] The present invention includes a bushing for making fibers from amolten material comprising at least one sidewall and a tip plate ororifice plate through which molten glass flows to form the fibers, and afirst screen having a generally uniform hole size and density spacedabove said tip plate, said first screen having holes therein and beingattached to said sidewall, the improvement comprising a second screenhaving holes therein and lying on top of said first screen, at leastsome of the holes in said second screen aligning with holes in saidfirst screen, the area of the holes per unit area of screen in saidsecond screen being less than the area of holes per unit area of screenin said first screen. The present invention also includes a method formaking fibers using such a bushing.

[0011] The present invention also includes the use of a screen orperforated plate (screen) having smaller hole sizes and/or holedensities in at least one area of the screen than the hole diameter orsize and/or hole density in the remainder of the screen with bushingsdesigned for non-channel positions to produce a bushing that works wellin channel positions on either side of the channel. The inventive screenlies on top of the conventional screen in the bushing. The resultantbushing is also a part of the present invention, as are the methods formaking bushings for a channel position. This invention greatly reducesthe number of new bushings and the amount of precious metal that must beinventoried in each plant or for each furnace.

[0012] This bushing has a first screen welded to the sidewalls andendwalls of the bushing, or to the flange of the bushing, in a normalway, the first screen having a uniform hole size and hole density in thearea of the screen containing holes or openings, and a second screenlying on top of the first screen, the second screen having a non-uniformhole size and/or hole density with the hole size and/or hole density ina center portion being smaller than the hole size and/or hole density inend portions of the screen. The second screen does not have to be weldedto any part of the bushing, but it can be tacked, pinned, riveted orotherwise attached in one or more places to keep it from moving and tomaintain acceptable alignment of the holes in the first and secondscreens as they sag with age.

[0013] The bushing of the present invention is made by welding a screenhaving a non-uniform hole pattern, hole density and/or hole diameterinto a standard bushing, or by laying a screen having a non-uniform holepattern, density and/or hole size on top of a conventional screen of aconventional bushing. In all embodiments of the present invention, thepercent of hole area of top or second screen, based on the total area ofthe top screen, is significantly smaller than the percent of hole areaof the first or bottom screen, based on the total area of the first orbottom screen. By significantly smaller or lower is meant at least about5 percent, preferably at least about 10 percent, and most preferably atleast about 20 percent, such as 25 percent, 30 percent or more.

[0014] When the inventive screen is not symmetrical from end to end,which it can be, the screen can be laid in face up to make a bushing fora channel position on one side of the channel or flipped over and laidin face down to make an inventive bushing for a channel position on theother side of the channel. Thus, in one embodiment of the presentinvention, only one kind of bushing and one kind of additional lay-inscreen need be inventoried. This reduces the amount of precious metal innew parts inventory at a plant or for each furnace substantially whilesolving the tip plate temperature gradient problem on channel, andsometimes second, positions.

[0015] The present invention also includes a method for making fibersfrom molten material using conventional fiber making processes exceptfor the use of one or more of the bushings of the present invention inthe manner described in detail below. When the term hole density is usedherein, it means the number of holes per square inch of screen area inthe portion of the screen being described. While round holes arepreferred and are used to describe the present invention, other shapesof holes can be used in the present invention. In some embodiments, thepercent of hole or open area in the central or center portion of thescreen of the invention is significantly less than the percent of holeor open area in the remainder of the screen. Hole density is directlyproportional to percent open area in the portion of the screen beingdefined. The hole size, as used herein to describe the invention, meansthe diameter of the holes in the screen, or a portion of the screen, orthe area of the holes, unless otherwise defined.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE INVENTION

[0016]FIG. 1 is a vertical cross section of a portion of a typical fiberglass furnace showing a glass melting furnace or tank, a channel, fourbushing legs and numerous spaced openings in the bottom of each leg forbushings, or positions for bushings.

[0017]FIG. 2 is a schematic view of a typical conventional tip typebushing for making continuous fiber from a molten material and havingmounted therein a conventional bushing screen.

[0018]FIG. 3 is a plan view of a conventional bushing screen having auniform hole size and density.

[0019]FIG. 4 is a schematic of a conventional marble or pellet meltbushing having a bushing screen with a uniform hole size and density.

[0020]FIG. 5 is a crossectional perspective view of a conventional tiptype bushing, having a conventional screen plate having a uniform holesize and hole density, that was modified with the present invention toproduce the preferred embodiment of the invention.

[0021]FIG. 6 is a plan view of a bushing screen design according to thepresent invention for use in the channel positions on either side of achannel in bushings whose length runs parallel to the length of thebushing leg.

[0022]FIG. 7 is a plan view of a bushing screen design according to thepresent invention for use in the channel position on either side of achannel on bushings whose length runs perpendicular to the length of thebushing leg.

[0023]FIG. 8 is a plan view of another bushing screen design accordingto the present invention for use in the channel position on either sideof a channel on bushings whose length runs perpendicular to the lengthof the bushing leg.

[0024]FIG. 9 is a perspective view of a bushing using a screen accordingto the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0025] In the direct melt method for making glass fiber, a glass meltingtank converts glass batch to molten glass and conditions the glass toremove most of the entrained gasses and to reduce the temperature belowthat used to melt the batch. Molten glass is removed from the meltingtank or refining chamber through a submerged throat and into one or morechannels which confines and transports the glass towards bushing legs inwhich a plurality of spaced apart bushings converts the molten glassinto glass fiber. While almost any glass can be made into fiber, themost commonly used is well known E type glass which is a lime-aluminaborosilicate. It is also known to use sodium boro-silicate glasses tomake fiber. The present invention is usable with any molten material,including any organic materials and any glass compositions that can bemade into a fiber using a fiberizing bushing.

[0026] Referring to FIG. 1, which is a horizontal cross section takenbelow the glass line of a portion of a conventional direct glass meltingand fiberizing system, the batch is melted in a glass tank 2 and isrefined in the downstream portion of the tank 2. Refining includeslowering the temperature of the molten glass significantly below themaximum temperature of the molten glass. To thoroughly melt batchcompletely at a competitive rate, it is necessary that the molten glassreach a temperature several hundred degrees hotter than the fiberizingtemperature of the glass. Some of this excess temperature can be removedin the refining end of the melting tank, but the temperature of themolten glass is still much too hot to fiberize when it enters one ormore channels 4.

[0027] While FIG. 1 shows only one channel 4, that channel can splitinto two or more channels, or two or more channels can emerge from thetank 2. The channel 4 confines the molten glass on its journey to one ormore bushing legs 6 and 8. Although the molten glass loses temperatureas it runs through the channel 4, at least when pulling a furnace at ornear the maximum rate, the temperature of the molten glass is still toohot for good fiberization. When this condition exists, the glass in thecenter of the flow tends to be the hottest and runs faster than thecooler glass closer to the walls and the bottom of the channel. Thishottest glass tends to dive into the first, and sometimes the second,bushing positions 12 and 16 in the bushing legs. FIG. 1 shows two waysthat bushings are typically oriented in the bushing legs 6 and 8. Inlegs 6, the bushing length is parallel to the length of the leg and inlegs 8, the bushing length is perpendicular to the length of the leg.While two different orientations are shown here on the same fiberizingoperation or furnace, normally the bushings would be oriented in thesame direction in each leg of a fiberizing operation or furnace.

[0028] Mounted below each leg and spaced apart are a plurality ofconventional bushings, such as shown in FIGS. 2 and 3, for convertingthe molten glass into fibers. These bushings 18 include a flange 20 atthe top of the bushing for sealing to a refractory bottom of the legs 6and 8 and are centered under vertical orifices that measure about 16.5to about 20 inches long and about 1-3 inches wide. The molten materialsuch as glass flows down into the bushings 18 through these orifices.

[0029] Referring again to FIGS. 2 and 3, conventional bushings 18 alsoinclude a tip plate or orifice plate 22, usually having a plurality oftips 24 thereon. Some bushings do not have tips 24, but fiberizedirectly from the orifices in the orifice plate 22. The tip plate 22 iswelded to bottom edges of sidewalls 26 and endwalls 28. Top edges of thesidewalls 26 and endwalls 28 are welded to the flange 20. A water cooledtube loop 30 is mounted under the flange to freeze the molten materialand form a seal with the refractory bottom of the legs 6 and 8. Thebushing 18 is normally made from platinum or a platinum-rhodium alloy inwhich the major portion is platinum when used to fiberize glass andother corrosive materials. The bushings are normally heated, usuallyelectrically heated by their own resistance with power cables (notshown) connected to terminal ears 32 on each end of the bushing. Theentire bushing assembly is mounted in an insulating refractory castablein a metal frame in a known manner. The metal frame connects to asuperstructure of the fiberizing operation or furnace to hold thebushing 18 in place.

[0030] The conventional bushing 18 shown in FIGS. 2 and 3 also containsa screen or screen plate 34 whose edges are usually welded to the insideof the sidewalls 26 and the endwalls 28, but the screen 34 can also bean integral part of the flange 20, or can lie over the flange 20. Thescreen plate 34 is spaced above the tip plate 22 by varying amounts, buttypically the bottom surface of the screen 34 is about 1.9375 inchesabove the top surface of the tip plate 22. This spacing can be larger orsmaller, and it can change with operating age since the screen usuallyoperates at a higher temperature than the tip plate and sags more. Thescreen plate has a plurality of holes or orifices 36 and is typicallyabout 0.09-0.15 inch thick. The purpose of the screen is to catch anylarge pieces of refractory or other unmelted debris in the moltenmaterial and prevent them from getting to the tip plate 22 where theywould interfere with molten glass getting to the tips, and to conditionor homogenize the temperature, viscosity and chemistry of the glass.Typically the holes in a conventional screen are about 0.053-0.057inches in diameter and the hole density is about 80-120 holes per squareinch.

[0031] The upstream end of the first orifice in each leg 6 or 8 istypically about 12-30 inches from the edge of the channel, dependingupon the leg design and the orientation of the bushing in the leg. Thehottest glass, and highest velocity glass, running into the legs 6 and 8tends to dive into the center portion of the first position in the leg,and on some legs also into the center of the second position. The hotstreak of glass flows right on through the conventional bushing screenand down to the tips. The colder glass surrounds this hot glass suchthat the tips at the ends of the bushings in the channel positions, oraround the periphery of the bushings when the bushing length isperpendicular to the leg, run colder. This is not desirable because thetips on the outer periphery tend to run colder anyway because of thecool air flowing into the fiber array. When the bushing is heated up tomake the tips in the outer periphery run better, then the center of thebushing is too hot and runs fibers having a coarse fiber diameter orbreaks out.

[0032] The present invention solves these problems by changing the flowof glass through the screen plate to compensate for the differenttemperatures of molten material like glass flowing onto the screenplate. FIG. 5 is a cross section of a prospective view of a portion of aconventional bushing for making glass fiber. This bushing is like thebushing shown in FIGS. 2 and 3 and one type of bushing used in thepresent invention. In addition to the elements shown in FIGS. 2 and 3and described above, this figure shows internal support ribs 38 whichare welded to the top of the tip plate 22 to retard sagging of the tipplate 22 due to the high operating temperature and the head of moltenglass pressing down on the tip plate. This figure also showsconventional finned water cooled tubes 40 below the tips for cooling thetips and the molten glass forming the fiber. The center tube 42 has twofins and a refractory piece 44 setting on and running along the tube 42between the two fins to support the center of the tip plate along itslength. The cooling tubes are supported by conventional hardware notshown here. Note that the internal support ribs 38 do not impede orcontain the molten glass as they do not extend to the screen plate andhave numerous openings along the bottom portion of the rib to allowmolten glass to flow past the support rib 38.

[0033] While the bushing described above is a cooling tube type bushing,the invention is equally useful on bushings that use other means ofcooling such as well known blade like fins, well known forced aircooling, or only ambient air cooling. The method of cooling the tips,orifice plate and molten glass emerging from the tips or orifices in theorifice plate is not critical to the present invention.

[0034]FIGS. 6, 6A and 6B show a preferred screen according to thepresent invention. The screen 46 has a low flow center portion 48 whichprovides more resistance to flow than a first end portion 50 and anopposite end portion 52 which are medium flow portions. This screen isfor channel position bushings which are mounted in the legs with thelength of the bushing running parallel with the length of the leg. Inthis embodiment of the present invention the holes are all about 0.55inch in diameter, but the hole densities are different. The hole densityin the center portion 48 is about 20 holes per square inch while thehole densities in the two end portions are about 60 holes per squareinch. The difference between the first, or channel, end portion 50 andthe opposite end portion 52 is that the first hole portion 50, which ispositioned to be closest to the channel 4, is smaller than the oppositeend portion which is farthest from the channel 4 when mounted in abushing and the bushing is installed in a leg. The dimension of thefirst end portion 50, along the length of the screen 48, is about 1.33inch, the length of the center portion 48 is about 9.12 inch and thelength of the opposite end portion 52 is about 2.05 inches in the mostpreferred embodiment. A band of the screen around the periphery is freeof holes. The end portions 54 of the band are preferably about 0.27 inchwide and the side portions 56 of the band are preferably about 0.37inch, but other band widths would be suitable.

[0035]FIG. 6A is a plan view of a portion of the end portions 50 and 52of the screen 46 showing how the holes in the end portions are arranged.The centers of the holes 58 preferably line up in both the length andwidth directions of the screen 46. The distance between the center linesof the rows of holes 58 in the length direction is preferably about 0.1inch and the distance between the center lines of adjacent rows of holesin the screen width direction is preferably about 0.19 inch, but otherhole arrangements and spacings can be used to achieve similar percentopen area in the end portions 50 and 52 of the screen 46. Preferably,the percent open area in the two end portions 50 and 52 of the screen 46is about 13.5 percent, but can be within the range of about 10 percentand about 16 percent.

[0036]FIG. 6B is a plan view of a portion of the center portion 48 ofthe screen 46 showing how the holes 60 in the center portion arepreferably arranged. Compared with the rows of holes in the end portions50 and 52, every third row of holes in the width direction of the screenare missing. Also, in the rows of holes in the width direction, thecenters of the holes 60 are preferably twice as far apart. Also, theholes in each row in the width direction are offset from the holes inthe adjacent row of the center portion 48. The distance between thecenters of the holes in a row in the width dimension of the screen 46 ispreferably about 0.2 inch, but other hole arrangements and spacings canbe used to achieve similar percent open area in the center portion 60 ofthe screen 46. Preferably, the percent open area in the center portion48 of the screen 46 is about 4.2 percent, but can be within the range ofabout 2.5 percent and about 6 percent.

[0037] The thickness of the screen 46 can be as thin as the capabilityof the equipment used to make the screen. The screen described in FIG. 6had a thickness of about 0.009 to about 0.015 inch, preferably about0.009 to about 0.011 inch. With the right equipment, a thinner screencan be made and will be suitable. The rate of flow of molten materialthrough the screen can also be controlled by using different screenthicknesses, i.e. a thicker screen in the low flow portion of the screenthan the thickness of the medium flow portions of the screen. Thismethod of modifying the flow through the screen is not preferred whenmaking fibers requiring precious metal screens because of the high costof precious metal. The bushing screen can be made by drilling, punchingor otherwise making holes, slits or other openings in a precious metalsheet, or by weaving precious metal wire in a known manner toproduce-the openings prescribed above.

[0038] The screen of the invention can be used in a bushing of the typeshown in FIGS. 2, 3 and 5 in place of the conventional screen 34 inwhich case the screen 46 would be welded to the sidewalls and endwallsof the bushing in the conventional manner. The bushing cannot be turnedend for end to use on either side of the channel 4 because of auxiliaryequipment used with the bushing. The screen 46 can be flipped over orturned end for end, or both, to make bushings for the channel positionson the opposite side of the channel 4, keeping the smaller medium flowend portion 50 closest to the channel 4 when the bushing is installed.This mode of the invention corrects the poor efficiency of the channelpositions and fiber diameter variation problems when using conventionalbushings, but requires that three different types of bushings beinventoried, the conventional type and a different type for each side ofthe channel.

[0039] The preferred mode of the invention eliminates much of theadditional inventory by requiring only conventional bushings and one ormore screens 46 as shown in FIG. 6. When preparing a bushing for achannel position, the screen 46 is simply placed inside the conventionalbushing to lay on top of the conventional screen 34, as shown in FIG. 9.To economize on precious metal, the length of the screen 46 is shorterthan the length of the conventional screen 34 to avoid placing blankmetal over blank metal, since screen 34 also has a blank band around itsperiphery that is similar to the blank band around the screen 46.

[0040] When the screen 46 is placed into the bushing 18, making sure theshort end portion 50 is on the channel end of the bushing, the screen 46is centered on the screen 34 and slight adjustment is made to line upthe holes in the screen 46 with holes in the screen 34 as well aspossible. The screen 46 is the same on the top as the bottom so it canbe flipped end over end, or turned around in a horizontal plane, ifnecessary, to place the short medium flow end portion 50 in the properend of the bushing. The screen can be left loose; but to insure itdoesn't shift during transporting to the furnace leg and installing, thescreen 46 can be secured to the screen 34 at each end with a few tackwelds, rivets or pins. This also helps maintain the alignment desiredbetween the holes in the two screens.

[0041]FIGS. 7 and 8 show screens according to the present invention foruse in channel position bushings oriented in a leg with the length ofthe bushing running perpendicular to the length of the leg, likepositions 16 in FIG. 1. Screen 62 has a low flow rate portion 64 locatedin the symmetrical center of the screen 62, and a medium flow rateportion 66 completely surrounding portion 64. The screen 62 also has aperipheral narrow band adjacent the outer edges (not shown) where thereare no holes like that of screen 46 shown in FIG. 6.

[0042] Usually the hot streak of molten material like glass is notcentered in the leg when it arrives at the channel position, but insteadis offset to the downstream side from the furnace tank, having justturned almost 90 degrees to enter the leg. Therefore, a more preferredscreen design for this type of bushing orientation is shown in FIG. 8.In this embodiment of the invention, a screen 68 has a low flow centralportion 70 that is not symmetrically located in the center of the screen68, but is offset towards the side of the leg that is downstream fromthe furnace tank and can also be offset in the direction of the upstreamend of the leg. Describing this in more detail, a medium flow portion 72of the screen surrounds the low flow center portion 70 in such a waythat dimension X in FIG. 8 is greater than dimension Y and dimension Ris greater than dimension S. In this screen 68, the shorter medium flowend portion 72 represented by dimension S would be closest to the legsidewall that is furthest downstream from the furnace tank and thenarrower medium flow side portion 72 represented by dimension Y would beclosest to the channel 4, closest to the upstream end of the leg, whenthe bushing containing this screen is installed in the leg.

[0043] In the embodiments of the invention shown in FIGS. 7 and 8, thehole sizes, hole densities and percent open areas in the conventionalscreen and the low flow portion and medium flow portion of the newscreens are the same as described for the screen shown in FIG. 6. Sincethe size of the furnace, the width of the channel and legs, the designof the entrance to the legs, the exit temperature of the moltenmaterial, the pull rate on the furnace and many other factors affectwhere the optimum location of the low flow portion of the screen shouldbe located for optimum performance, some experimentation using the abovedisclosure as the guide is necessary for optimization.

[0044] The present invention has other uses to give a standard designbushing that is already built, or even in place making fiber, greatercapability or better efficiency. Two of these applications includeallowing a bushing designed to make coarse fiber to also make fine fiberwithout rebuilding and allowing a bushing designed to run one kind ofglass also run another kind of glass having a completely differentfiberizing temperature and temperature-viscosity curve.

[0045] A bushing designed to make 16 micron diameter fiber from a hotmolten material like molten glass has orifice size in the tips to allowmaximum flow rate for whatever fiber pulling speed is desired to achievemaximum productivity. This type of bushing will not run finer fiber like13 micron and particularly 10 micron diameter at acceptable pull ratesand efficiency because the bore in the tips is too large and the flowrate at fiberizing temperature through each tip is too great for thefiber pulling speed capability of the fiber pulling equipment,especially when making direct chop fiber. If customer needs switch fromneeding more fine fiber and less 16 micron or larger fiber, it has beenvery expensive to adjust manufacturing equipment to meet such a need.Normally a bushing lasts for 6 to 12 months or more and changing abushing prematurely is very expensive. Also, once a direct melt bushinghas been installed and run, removing it almost always damages thebushing to the extent that it must be cleaned up, remelted and rebuiltbecause it cannot be reused any other way. This is very expensive.

[0046] The present invention allows a bushing designed for 16 micronfiber to run 13 or even 10 micron fiber at close to optimum pull ratesand efficiency. This embodiment of the invention is applicable to new,not yet installed direct melt bushings and to marble melt bushings, newor old and running. In this embodiment of the invention a new screen,according to the invention, is laid on top of the existing conventionalscreen to reduce the flow of molten material through the screen and thusto reduce the head of glass on the tip plate to that suitable to allowthe tips with the larger bore to run fine fiber at a normal fiberpulling speed. In this embodiment, the new screen, to be laid on top ofthe conventional screen, will have a hole density and hole diameter thatproduces a substantially greater resistance to flow of the moltenmaterial than the conventional screen. The hole diameter and holedensity can be uniform across the screen or it can be non-uniform tocorrect cold or hot end problems or other tip plate temperature profileproblems if they exist by directing more or less glass into a problemarea than into the non-problem areas to improve the profile. It ispossible, with certain designs of bushing legs, that a woven wire screenaccording to the present invention can be inserted into the bushingwhile running by passing a small screen according to the presentinvention through the opening in the bottom of the leg and onto thecenter of the conventional screen in the bushing.

[0047]FIG. 4 shows a conventional marble or pellet melt bushing having atip plate 74 with tube tips 76 spaced apart and welded to the tip plate74 around holes 78 in the tip plate. The tip plate 74 is welded tosidewalls 80 and endwalls 82 and also to internal supports 84. A bottomsurface of a screen plate 86 is welded to the tops of the sidewalls 80and endwalls 82 and to the internal supports 84. A curved enclosure 88,open at the top, is welded to a top surface of the screen 86. The screen86 has holes or orifices 90 therethrough in the portion of the screenenclosed by enclosure 88. The bushing is made from precious metal likewell known platinum-rhodium alloys when used to fiberize hightemperature molten materials like glass and is heated by its ownresistance by feeding electrical power to the terminals 92 on each endof the bushing. Optional braces 94 can be used to support the side wallsof the enclosure to keep them apart during use. This bushing is mountedin a known way by surrounding it with insulating refractory in astainless steel frame suitable for mounting to steel superstructure.

[0048] Glass pellets, marbles, beads, etc. are fed to this bushing afterit is heated up and the screen plate 86 gets very hot and melts theglass. When the molten glass becomes hot enough that its viscosityallows it to run through the holes 90, the molten glass flows throughthe screen and on to the tip plate 74 where it flows through holes 78and tips 76 to be fiberized in a known way. This bushing is designed notonly to make particular fiber diameters, but also to operate on aparticular glass, i.e. one having a particular temperature-viscosityrelationship. To enable the bushing to melt the cold glass marbles, etc.at a good production rate, the screen plate 86 becomes much hotter thanthe fiberizing temperature of the molten glass. The size and density ofthe holes 90 are critical to optimizing the melt rate and fiberizingproductivity of any particular fiber diameter and any particular glass.Therefore, a bushing designed for coarse fiber and a particular glass isnot practical for making fine fiber or even coarse fiber from asignificantly different glass composition.

[0049] The present invention allows a marble melt bushing designed formaking coarse fiber to be easily and quickly modified, withoutrebuilding the bushing, so that it can be used to efficiently make finerfiber. A screen plate having a substantially lower flow rate than theconventional screen plate 86 welded into the bushing and a flow rateappropriate to the desired fiber diameter and desired fiber pullingspeed is inserted into the bushing and laid on top of the existingscreen plate 86. This can be done by first stopping the feed of coldglass to the bushing, heating the bushing to about 50-100 degrees F.hotter than normal running temperature, draining as much of the moltenglass as will run out, cooling the bushing, removing it to the workshopto install the screen. When the braces 94 are not used, the new screenaccording to the present invention can usually be installed while thebushing is in place and hot, though cooled some from operatingtemperature, after some of the molten glass has been drained from thebushing. When it is desired to use the bushing to again make coarsefiber, the lay-in screen of the present invention can be removed usingthe same procedure as used to install the new screen.

[0050] The present invention can also be used to allow a marble meltbushing designed for melting and fiberizing one type of glass having aparticular temperature-viscosity curve to also melt and fiberize asubstantially different glass having a higher melting temperature and adifferent temperature-viscosity curve than the glass for which thebushing was originally designed. A bushing for melting a highertemperature glass normally will have a screen having a lower flow ratethrough the screen by having smaller holes or a lower hole density orboth. The reason is that as the melt temperature of the glass goes up,the desired screen temperature to get good melt rate, combined with theviscosity characteristics of the molten glass, tend to cause too rapid aflow rate of molten glass through the holes in the conventional screenand colder glass than desired to run through the screen. This can becorrected by reducing the hole density or preferably by reducing thesize of the holes, and sometimes also the hole density. On a marble orpellet melt bushing the new screen can be installed while the bushing ishot by stopping the feed and letting all of the marbles, etc. meltbefore laying in the screen. It is most desirable to let the moltenglass above the original screen to drain down through the screen beforeinserting the screen of the present invention.

[0051] Other modifications of the present invention will be obvious tothose skilled in the fiber making art. Also, other uses of the presentinvention on bushings for fiberizing molten material will be obvious tothose skilled in the art having the benefit of the above disclosure, andthose uses are intended to be within the scope of the following claims.

1. In a bushing for making fibers from a molten material comprising atleast one sidewall and a tip or orifice plate through which moltenmaterial flows to form the fibers, and a first screen having a generallyuniform hole size and density spaced above said tip plate, said firstscreen being attached to said sidewall, the improvement comprising asecond screen lying on top of said first screen, said second screenhaving a significantly lower percentage of hole area, based on the totalarea of the second screen, than the percentage of hole area of saidfirst screen, based on the total area of the first screen.
 2. In abushing for making fibers from a molten material comprising at least onesidewall and a tip plate or orifice plate through which molten materialflows to form the fibers, and a screen having a plurality of holestherethrough and mounted on the interior of the bushing and spaced abovethe top of the tip plate or orifice plate, said screen having holestherein and being attached to said sidewall, the improvement comprises agenerally mid or central portion of the screen having a hole area perunit area of screen that is significantly smaller than the hole area perunit area of screen of end portions of the screen on either side of themid or central portion.
 3. The bushing of claim 1 wherein said materialis glass and said bushing, including the screen, is made from a preciousmetal or precious metal alloy with the major portion of said metal beingplatinum.
 4. The bushing of claim 2 wherein said material is glass andsaid bushing, including the screen, is made from a precious metal orprecious metal alloy with the major portion of said metal beingplatinum.
 5. The bushing of claim 1 wherein the percentage of hole areain said second screen is at least 10 percent less, per unit area, thanthe percentate of hole area in said first screen.
 6. The bushing ofclaim 5 wherein the percentage of hole area in said second screen is atleast 20 percent less, per unit area, than the percentate of hole areain said first screen.
 7. The bushing of claim 6 wherein the percentageof hole area in said second screen is at least 30 percent less, per unitarea, than the percentate of hole area in said first screen.
 8. Thebushing of claim 2 wherein the hole area per unit area of screen in saidcentral portion is at least 10 percent less than the hole area per unitarea of said end portions.
 9. The bushing of claim 8 wherein the holearea per unit area of screen in said central portion is at least 20percent less than the hole area per unit area of said end portions. 10.The bushing of claim 9 wherein the hole area per unit area of screen insaid central portion is at least 30 percent less than the hole area perunit area of said end portions.
 11. A screen for a fiberizing bushinghaving a plurality of holes therethrough, said screen comprised of a midor central portion and two end portion, said mid or central portionhaving a hole area per unit area of the central portion that issignificantly less than the hole area of the end portions per unit areaof the end portions.
 12. The screen of claim 11 wherein saidsignificantly less is at least 10 percent.
 13. The screen of claim 12wherein said significantly less is at least 20 percent.
 14. The screenof claim 13 wherein said significantly less is at least 25 percent. 15.The screen of claim 14 wherein said significantly less is at least 30percent.
 16. A method of making fibers from a molten material whereinsaid molten material is heated in a bushing comprising at least onesidewall and a tip or orifice plate through which molten glass flows toform the fibers, and a first screen having a generally uniform hole sizeand density spaced above said tip plate, said first screen beingattached to said sidewall, the improvement comprising using a secondscreen lying on top of said first screen, said second screen having asignificantly lower percentage of hole area than the percentage holearea of said first screen.
 17. The method of claim 16 wherein saidmaterial is glass and said bushing is made from precious metal or alloysof precious metal containing a majority of platinum and wherein saidsignificantly lower is at least about 10 percent lower.
 18. The methodof claim 17 wherein said significantly lower is at least about 20percent lower.
 19. The method of claim 18 wherein said significantlylower is at least about 30 percent lower.
 20. The method of claim 16wherein said bushing is used to make fibers having a smaller fiberdiameter than the bushing containing said first screen was designed tomake.
 21. A method for forming fibers from a molten material in achannel position of a multi-bushing fiberizing operation comprising atleast one sidewall and a tip plate or orifice plate through which themolten material flows to form the fibers, and a screen spaced above saidtip plate having a plurality of holes therein, said screen beingattached to said sidewall, the improvement comprising a bushing screenin said bushing having a hole area per unit of screen area in a centerportion of the screen that is significantly less than the hole area perunit of screen area in at least one end portion of the screen.
 22. Themethod of claim 21 wherein the hole area per unit of screen area in thecentral portion of the screen is less than at least about 30 percent ofthe hole area per unit of screen area in said at least one end portionof said screen.
 23. In a method for forming fibers from a moltenmaterial in a channel position of a multi-bushing fiberizing operationcomprising at least one sidewall and a tip plate or orifice platethrough which the molten material flows to form the fibers, and aconventional screen spaced above said tip plate and having a pluralityof holes therein, the conventional screen being attached to saidsidewall, the improvement comprising using a second screen having a holediameter and/or hole density in the central portion of the screen thatis significantly less than the hole diameter and/or hole density in theend portions of the screen lying on top of the conventional bushingscreen.
 24. The method of claim 23 wherein the hole size and/or holedensity of said central portion of said second screen is such that thepercentage of hole area in said central portion is at least about 10percent less than hole area percent of the end portions.